Good afternoon,
In 2016 we installed a self engineered system that works on the output frequency shift of a Victron inverter (3 Quattros) to heat the water.
We installed a Trumeter APM frequency meter, an Akytec TRM-210 PID process controller and 3 Fadisel R-27 5000W choppers connected to 3x2000W coils in one of the two 1000 litre “hot” tanks.
The PV field inverters we programmed to reduce power only above 51Hz, instead of the usual 50,20Hz.
The PID controller we programmed to keep the frequency in between 50,06 and 50,08Hz, so all the excess PV power will go in the hot water and the PV inverters keep producing the maximum possible.
This starts normally around 1300-1500 till sun down, as then our batteries are getting full and the Quattros want to reduce the PV power by raising the frequency, but instead water is heated.
We placed an extra APM frequency meter in the kitchen, so the “ladies” there would know when to switch on washing machines as these meters can be programmed to change colour, “red” not wash, “green” wash, even flashing if needed.
The results are in after 8 years of use, 1500 KWh in average each year, this with 8 KWp PV panels which generated 9000 KWh a year in average, meaning, at about 330 Euro savings a year if we had to buy that energy.
Just when we mounted this installation the Ohm-pilot was invented (2017-isch), but this one had/has only 1 proportional output and the other 2 are switched fully on/off.
The inverter phase connected to that proportional regulated coil is doing 80-90% of the work all year long, and not keeping the 3 phases in balance, for single phase no issues there.
Also it cannot be used without “again” a smart meter to see that nothing goes to the grid…
Then the MY-PV Thor was invented (2018-isch), mainly the 9S (3 phase) was of our interest, 3 phases fully proportional, frequency control and can be used with a smart meter, that’s cool we thought.
But, we lost quickly interest in both machines and even in our DIY made one…runs fine btw.
Why, easy, this amount of 1500 KWh for hot water was actually only a drip into the ocean, this as we also have 16KW vacuum tubes, generating 48000 KWh a year average and besides that, we use 2 pieces 75KW wood incinerators who deliver 40000 KWh a year average.
We thought that we “lost” way more energy due to the reduction of our panels, but it is not that much.
So, the safed energy with such device is not worth the investment, as it can take years of use to get it back in the pocket, also, it generates/saves only 1.5% on our hot water use per year in average.
Basically all 3 systems as above described are around a 1500-2500 Euro to build/install by yourself as it’s not just the controls part, but also the water coils, cabling, fuses, etc.
Well, with a return of 330 Euro per year on such investment in our case, that takes about 5-8 years before it starts to make a profit…maintenance and/or defects not calculated.
Over dimensioning the PV field was opted by us, but then again, more investment, and the risc that the frequency starts to limit the PV field inverters as well, and to have to set the clocks once a week correct…no…
For that investment it would be better to buy extra/bigger batteries or 1-2 extra vacuum tube panels is our conclusion as we do not deliver to the grid.
One big advantage of the TRM-210 is that it will keeps the frequency between 50,06 and 50,08Hz, unlike the PV inverters, which reduce only according a certain amount of power against frequency, having to set clocks all the time due to the high frequency, never late for work do…
Just for that I would advise to install such system.
Regards, Jeroen.
that should not be a criticism at all 
That’s what this is supposed to be for